Extrusion blow molding is a common process for creating hollow articles from polymeric materials. A typical extrusion blow-molding manufacturing process involves: 1) melting the resin in an extruder; 2) extruding the molten resin through a die to form a tube of molten polymer (i.e. a parison) having a uniform side wall thickness; 3) clamping a mold having the desired finished shape around the parison; 4) blowing air into the parison, causing the extrudate to stretch and expand to fill the mold; 5) cooling the molded article; and 6) ejecting the article of the mold.
In order to form good quality containers that have uniform side wall thickness and to prevent tearing of the parison during expansion (i.e. blowing), the polymer extrudate must have good molten dimensional stability, also known as melt strength. A material having good molten dimensional stability (i.e. high melt strength) has a tendency to resist stretching and flowing as a result of gravitational force when in the softened or molten state. Excessive stretching of the extrudate parison causes the walls to become too thin. This leads to lack of uniformity in the wall thickness. Thin walls also have a greater tendency to tear under the influence of the air pressure being used to expand the extrudate into the mold walls.
In extrusion blow molding, the polymer melt usually is extruded vertically from a die into a parison, whereby melt strength can be determined by measuring the vertical length of an extrudate after a certain amount of time to determine the extent to which the extrudate stretches or “sags”. When measuring sag, the extruder output and die gaps are fixed, whereby a given volume and weight of material is extruded over a fixed length of time. Given these conditions, the extrudate of a polymer with low melt strength will be long and thin. In contrast, the extrudate of a polymer with high melt strength will be short and thick. In addition, the sag of the extruded parison is directly related to the weight of the parison, whereby larger and heavier parisons will have a greater tendency to sag. Therefore, higher melt strength materials are required to allow larger and heavier parisons, e.g., for making larger bottles, and to maintain their shape. The higher the melt strength, the larger the bottle that can be produced.
Since melt strength is related to slow flow induced primarily by gravity, it can be related to viscosity of a polymer measured at a low shear rate, such as 1 radian/second. Viscosity can be measured by typical viscometers, such as a parallel plate viscometer. Typically, viscosity is measured at the typical processing temperature for the polymer, and is measured at a series of shear rates, often between 1 radian/second and 100 radian/second. In extrusion blow molding, the viscosity at 1 radian/second at processing temperatures typically needs to be above 30,000 poise in order to blow a bottle. Larger parisons require higher viscosities.
Melt strength, however, only defines one of the processing characteristics important in extrusion blow molding. The second important characteristic is ease of flow at high shear rates. The polymer is “melt processed” at shear rates ranging anywhere from about 10 s−1 to 1000 s−1 in the die/extruder. A typical shear rate encountered in the barrel or die during extrusion blow molding or profile extrusion is 100 radians/second. These high shear rates are encountered as the polymer flows down the extruder screw, or as it passes through the die. These high shear rates are required to maintain reasonably fast production rates. Unfortunately, high melt viscosity at high shear rates can lead to viscous dissipation of heat, in a process referred to as shear heating. Shear heating raises the temperature of the polymer and the extent of temperature rise is directly proportional to the viscosity at that shear rate. Since viscosity decreases with increasing temperature, shear heating decreases the low shear rate viscosity of the polymer and thus its melt strength decreases.
Furthermore, a high viscosity at high shear rates, (for example as found in the die) can create a condition known as melt fracture or “sharkskin” on the surface of the extruded part or article. Melt fracture is a flow instability phenomenon occurring during extrusion of thermoplastic polymers at the fabrication surface/polymer melt boundary. The occurrence of melt fracture produces severe surface irregularities in the extrudate as it emerges from the orifice. The naked eye detects this surface roughness in the melt-fractured sample as a frosty appearance or matte finish as opposed to an extrudate without melt fracture that appears clear. Melt fracture occurs whenever the wall shear stress in the die exceeds a certain value, typically 0.1 to 0.2 MPa. The wall shear stress is directly related to the volume throughput or line speed (which dictates the shear rate) and the viscosity of the polymer melt. By reducing either the line speed or the viscosity at high shear rates, the wall shear stress is reduced lowering the possibility for melt fracture to occur. Although the exact shear rate at the die wall is a function of the extruder output and the geometry and finish of the tooling, a typical shear rate that is associated with the onset of melt fracture is 100 radian/sec. Likewise, the viscosity at this shear rate typically needs to be below 30,000 poise.
To couple all of these desired properties, the ideal extrusion blow molding polymer, from a processability standpoint, will possess a high viscosity at low shear rates in conjunction with a low viscosity at high shear rates. These attributes are also useful in other melt processes. For injection molding, the low viscosity at high shear rates will allow the polymer to easily flow into the mold. However, once flow has stopped and the shearing removed, the polymer rapidly becomes highly viscous so that the part can be quickly removed from the mold. For profile extrusion, a high viscosity at low shear rates maximizes melt strength while low viscosity at high shear rates minimizes screw motor load, pumping pressure, shear heating and melt fracture.
Fortunately, most polymers naturally exhibit at least some degree of viscosity reduction between low and high shear rates, known as “shear thinning”, which aids in their processability. Without the shear thinning, an extruder running a high melt viscosity polymer would require extremely high motor loads and/or very high melt temperatures, both of which can lead to polymer degradation and excessive energy consumption. The ideal polymer noted above would possess a high degree of shear thinning. Based on the preceding discussion, one definition of shear thinning important to the processes discussed in this invention would be the ratio of the viscosity measured at 1 radian/second to the viscosity measured at 100 radians/second. These viscosities would both be measured at the same temperature, typical of the actual processing conditions. This definition will be used to describe shear thinning for the purposes of this invention.
Unfortunately, certain polymers such as polycarbonates and polyesters such as poly(ethylene terephthalate) (PET) and poly(ethylene-co-1,4-cyclohexylene-dimethylene terephthalate) (PETG) have a very low degree of shear thinning as compared to polymers like PVC, polystyrene, acrylics and polyolefins. Since these other polymers suffer from one or more of their own disadvantages (e.g. cost, odor, clarity, toughness, chemical resistance), polyesters would be ideal alternative materials in similar applications if processing difficulties of polyesters could be overcome.
It is possible to increase the melt strength of a polymer by lowering the melt temperature but, since the high shear rate viscosity is also increasing, eventually the temperature will drop to the point where melt fracture appears. Decreasing the temperature also increases the degree of shear thinning so it does make it possible to process articles of up to a certain size, but the improved degree of shear thinning is usually insufficient to enable the production of large articles.
It is also possible to increase the melt strength and degree of shear thinning by increasing the molecular weight and molecular weight distribution of the polyester through solid state polymerization. Again, however, the improvement in shear thinning obtained by this method is usually insufficient to allow the production of large articles. Furthermore any polyester that can be solid stated is crystallizable, whereby it can not be processed at a temperature lower than its melting point. Certain solid stated polymers may also exhibit a phenomenon referred to as “unmelts”, wherein a portion of the solid stated pellets possess a very high melting point, or a very high viscosity whereby they do not disperse in the melt pool. The resulting pellet sized particles are easily observed in the parion and resultant bottle. These unmelts are an unacceptable visual defect. In order to eliminate unmelts, the material must be processed at higher temperatures, which often results in an unacceptable reduction in melt strength.
A linear polyester is defined as a polyester that is prepared from A-A and B-B monomers or A-B monomers or combinations of these with the correct stoichiometric balance. A-A monomers can represent dibasic acids such as terephthalic acid, isophthalic acid and B-B monomers can represent diols such as ethylene glycol and 1,4-cyclohexanedimethanol. An A-B monomer can represent p-hydroxybenzoic acid, etc. When the stoichiometry of these polymerizing systems is correct, linear high molecular weight polyesters are readily prepared. Diesters of the dicarboxylic acid can be used instead of the dicarboxylic acids and high molecular polyesters can be prepared by transesterification process.
It is possible to add a branching agent into the reactor so that the resulting polymer chain is no longer linear. Branching agents usually are defined by the number of functional groups attached and can take the form of an A3 or B3 molecule where A3 is a tricarboxylic acid or tricarboxylic acid ester and B3 is a triol. Also A2B and AB2 monomers can be employed to affect branching where A2B represents a monomer with 2 acid functional groups and 1 alcohol and AB2 represents a molecule of 1 acid functionality and 2 alcohol groups. Higher functionality branching groups can also be employed for this purpose including tetrafunctional groups, such as from pentaerythritol and phromellitic dianhydride. The science related to polyester branching is well known in the polyester art. Chain branching is one of the most common methods for improving the melt strength of a polymer, particularly polyesters. However, the use of branching agents can lead to unacceptable gel formation in the melt, especially if the branched material has been solid stated. A gel is nothing more than a point in the polyester where too much localized branching occurs, effectively creating a tightly interconnected network of chains that cannot be easily melted. This gel is present in the final molded/extruded part as an unacceptable visual defect. To minimize gelling, the branching agents are added at a low level with uniform dispersion throughout the reactor. Thus, a branched polyester is difficult to produce and the increase in melt strength is limited to the maximum amount of branching agent that can be added without gel formation.
Amorphous copolyesters comprising terephthalic acid (T) residues with different ratios of 1,4-cyclohexanedimethanol (CHDM) and ethylene glycol (EG) residues are well known in the plastics marketplace. As used herein, the abbreviation PETG is used for compositions wherein the diacid component contains or comprises terephthalic acid residues and the diol component comprises up to 50 mole percent CHDM residues with the remaining diol component being ethylene glycol residues. PCTG is used herein to refer to copolyesters wherein the diacid component comprises terephthalic acid residues and the diol component comprises greater than 50 mole percent CHDM residues with the remainder being ethylene glycol residues.
Neopentyl glycol (NPG, 2,2-dimethyl-propane-1,3-diol) is another common diol used in the preparation of polyesters. Similar to CHDM, NPG has been used in combination with EG and terephthalic acid to form useful amorphous copolyesters. However, the combination of NPG and CHDM as the sole glycol components of the copolyester has received minimal attention.
Several early references disclose polyesters containing both CHDM and NPG residues with terephthalic acid residues as the diacid compoent. Example 46 of U.S. Pat. No. 2,901,466 describes a copolyester prepared from CHDM and NPG residues that was solid stated to an IV of 1.06. The CHDM was described as being “75% trans”. The copolyester was reported to have a crystalline melting point of 289–297° C. The exact composition of this polyester was not disclosed, but the melting point of this polymer is not very different from that of pure poly(1,4-cyclohexylene-dimethylene terephthalate) (PCT, Tm=293° C.).
U.S. Pat. No. 3,592,875 discloses polyester compositions that contain both NPG and CHDM residues with an added polyol present for branching. U.S. Pat. No. 3,592,876 discloses polyester compositions that contain both EG, CHDM and NPG residues with the level of NPG residues limited to up to 10 mole percent. U.S. Pat. No. 4,471,108 discloses low molecular weight polyesters, some of which contain CHDM and NPG residues but which also contain a multifunctional branching agent. U.S. Pat. No. 4,520,188 describes novel low molecular weight copolyesters with mixtures of aliphatic and aromatic diacid residues with both NPG and CHDM residues present.
U.S. Pat. No. 4,182,841 describes a composition containing between 80 and 70 mole percent ethylene glycol and between 20 and 30 mole percent neopentyl glycol that also contains a polyfunctional modifying material, i.e., a branching agent. Terephthalic acid is the only acid used in the compositions. CHDM was not mentioned. U.S. Pat. Nos. 5,523,382 and 5,442,036 describe a branched copolyester suitable for extrusion blow molding. The copolymer contains ethylene glycol (EG) residues in addition to 0.5 to 10 mole percent of CHDM residues and 3 to 10 mole percent diethylene glycol (DEG) residues. The diacid component comprises terephthalic acid residues with up to 40 mole percent isophthalic acid (IPA) or 2,6-naphthalenedicarboxylic acid (NDA) residues. The branching agent preferably consists of trimellitic acid or anhydride. NPG is not mentioned.
U.S. Pat. No. 4,983,711 describes a branched copolyester composed of EG and CHDM residues and consisting of from 0.05 to 1 mole percent of a tri-functional branching agent, preferably trimellitic acid or anhydride. Preferred levels of the branching agent are from 0.1 to 0.25 mole percent. This patent discloses CHDM residue levels of 25 to 75 mole percent and is concerned with extrusion blow molding applications. The prevention of melt fracture is not mentioned. NPG is not discussed. U.S. Pat. No. 5,376,735 describes a branched poly(ethylene terephthalate) modified with up to 3 mole percent IPA residues for use in extrusion blow molding applications. A number of branching agents are mentioned including TMA.
U.S. Pat. No. 5,235,027 describes a branched co-poly(ethylene terephtalate) for extrusion blow molding. The PET contains from 0.5 to 5 weight percent IPA residues, 0.7 to 2.0 weight percent DEG residues, 300–2500 ppm tri- or tetra-hydroxyalkane residues, 80–150 ppm Sb, phosphorous at least 25% by weight of the amount of Sb, red and blue toner (not exceeding 5 ppm), and various branching agents with pentaerythritol being preferred. NPG is not discussed.
U.S. Pat Nos. 4,234,708, 4,219,527 and 4,161,579 describe branched and end capped modified PET polyesters for extrusion blow molding. A variety of chain branching agents (from 0.025 to 1.5 mole percent) and 0.25 to 10 equivalents of a non-ionic chain terminator are described for controlling reaction conditions and preventing gelling. NPG is not discussed. U.S. Pat. No. 4,398,022 describes a high melt strength copolyester consisting of terephthalic acid and 1,12-dodecanedioic acid residues along with a diol component comprising CHDM residues. No branching agent was utilized. Japanese Patent Publication JP 3225982 B2 discloses amorphous copolyesters said to be useful in the formulation of coating compositions for steel sheet. The disclosed copolyesters comprise a diacid component comprising mixtures of aliphatic and aromatic acid residues and a diol component comprising NPG and CHDM residues.